In many applications, in particular in projection systems, there is a need for light sources which are as small as possible while providing a high light-flux. Therefore, in recent years, high pressure gas-discharge lamps such as HID (High Intensity Discharge) or UHP (Ultra High Performance) lamps have become the preferred choice for such applications because of the small distance between the electrodes and the correspondingly short arc-length and high brightness. A stable arc-length is of utmost importance for the use of short arc HID-lamps in projection applications since a constant light flux is desired. Especially for the new ultra-short-arc versions of UHP, the flux-maintenance in modern projectors largely depends on sustaining short arc lengths for prolonged times. Measures typically employed to maintain the arc-length include electrode design and lamp driving schemes. In principle, lamp driving schemes stabilize the arc-length by an essentially controlled growing and melting of structures on the lamp's electrodes.
For example, in WO 2005/062684, a method is proposed in which specific combinations of different current wave-shapes and operating frequencies are used. In a first mode of operation, this is substantially achieved by means of a known block shape of the lamp current upon which current-pulses are superimposed, directly preceding a commutation of the current. In this first operation mode, the lamp is always driven with a low frequency. In a second mode of operation, the thin tip growing on the electrode end always has a molten front structure, even if the lamp current is free of current-pulses. In this second operation mode, the lamp is always driven at a higher frequency. A third mode of operation is chosen such that the electrode tip to be melted back is necessarily in the molten state. A disadvantage of this existing method is that it leads to frequent changes of the operating frequency of the lamp. Besides occasionally having a negative effect on arc-stability, such frequency changes also complicate the use of this type of lamp in modern projectors, where often the control of the display has to be synchronized to the operating frequency of the lamp. On the other hand, arc-stability issues sometimes make inevitable such changes in the operating frequency.